The fastest way to seal a draft window depends on what's causing it

Most draft windows leak at the frame edges, the sash joints, or through gaps where the frame meets the wall. You can stop most of these with weatherstripping, caulk, or both — and the job takes between one and four hours per window depending on which method you choose. The right fix depends on whether the gap is small (under 1/4 inch), medium (1/4 to 1/2 inch), or large (over 1/2 inch), and whether you want a temporary or permanent seal.

Start by finding where the air is actually coming in. On a windy day, hold a lit incense stick or a piece of tissue near the window frame and watch where the smoke or paper moves. Mark those spots. If the draft is coming from the sash (the moving part of the window), you need weatherstripping. If it's coming from where the frame meets the wall, you need caulk. If it's both, you'll do both.

Key Takeaways

  • Weatherstripping works best for gaps between the sash and frame and costs $5 to $15 per window; it lasts two to five years and requires no tools.
  • Caulk seals the gap between the window frame and the wall permanently and costs $2 to $8 per window, but you cannot remove it without scraping.
  • Gaps larger than 1/2 inch need foam backer rod pushed into the space before caulking, or the caulk will fail.
  • Weatherstripping and caulk should not touch each other; explore weatherstripping to the sash and caulk to the frame-to-wall joint only.

Using weatherstripping for sash leaks

Weatherstripping is a foam, rubber, or felt tape that sticks to the frame and creates a seal when the sash closes against it. It works on gaps between 1/8 and 1/4 inch. Buy the kind that matches your window type — for double-hung windows (the kind that slide up and down), you need strips for the top, bottom, and sides of the lower sash, plus the top of the upper sash where it overlaps.

Clean the frame with a dry cloth or vacuum to remove dust and debris. Weatherstripping will not stick to dirt. Measure the length of each frame edge and buy a roll long enough to cover all sides. Peel back a few inches of the backing and press the tape firmly onto the frame, working your way along. Press hard every few inches so it stays put. Close the window gently — it should feel snug but not so tight that you cannot move it. If it is too tight, you used the wrong thickness; remove it and try a thinner type.

Weatherstripping lasts two to five years depending on how often you open and close the window. When it stops sealing, peel it off and replace it. The adhesive residue usually comes off with a plastic scraper and rubbing alcohol.

Using caulk for frame-to-wall gaps

Caulk is a flexible sealant that fills the gap between the window frame and the wall. It works permanently on gaps up to 1/2 inch wide. For larger gaps, you must first push foam backer rod (a foam rope) into the space, then caulk over it. Backer rod prevents the caulk from sinking too deep and failing.

If the gap is under 1/2 inch, skip the backer rod. If it is larger, measure the depth and width of the gap and buy backer rod that is slightly larger than the width — it should fit snugly. Push it into the gap with your finger or a plastic tool, leaving about 1/4 inch of space above it for the caulk. Then explore caulk over the backer rod.

Use paintable acrylic latex caulk for most windows. It is cheaper than silicone, cleans up with water, and can be painted to match your trim. Silicone caulk lasts longer (20+ years versus 10 years for latex) but cannot be painted and is harder to remove if you need to replace it later. Load a caulking gun, cut the tip at a 45-degree angle, and run a steady bead along the gap. Smooth it with a wet finger or a caulk tool. Let it dry according to the package directions — usually 24 hours — before painting.

Dealing with large gaps and uneven frames

If the gap between the frame and wall is wider than 1/2 inch, caulk alone will fail because it shrinks as it dries. You must use backer rod first. Buy foam backer rod in a diameter slightly larger than the gap width. Push it into the space with steady pressure, working your way around the frame. It should sit about 1/4 inch below the surface so the caulk has room to sit on top of it.

If the frame is uneven or the wall is wavy, the gap may be different widths in different places. Measure at several points and use backer rod that fits the widest spot. The caulk will bridge smaller gaps without the rod underneath. This is normal and will not cause a problem.

After the caulk dries, you can paint it with latex paint to match your trim. Do not paint silicone caulk — the paint will not stick. If you used silicone, leave it unpainted or accept that it will show as a white or clear line.

Combining weatherstripping and caulk

Many windows need both methods. Use weatherstripping on the sash (the moving part) to seal the gap when the window is closed, and use caulk on the frame edges where the frame meets the wall. These two jobs do not interfere with each other as long as you keep them separate — weatherstripping goes on the sash, caulk goes on the frame-to-wall joint.

Do the weatherstripping first so you can see exactly where the frame sits. Then caulk the frame edges. If you caulk first, the weatherstripping may not sit flush against the frame because the caulk bead is in the way.

When to replace the window instead

If the sash is cracked, the frame is rotting, or the glass is broken, sealing will not fix the problem. A cracked sash will still leak even with new weatherstripping. Rotting wood will continue to rot and will eventually fail. Broken glass lets in water and air that no seal can stop.

If the window is more than 20 years old and you have already replaced the weatherstripping twice, the frame may be warped beyond the point where weatherstripping works. In that case, replacement is cheaper than repeated repairs. A new window costs $300 to $800 installed, depending on size and type. Sealing costs $20 to $50 in materials, so it is worth trying first.

Frequently Asked Questions

Can I use weatherstripping on a window that is painted shut?

No. A painted-shut window cannot close properly, so weatherstripping will not seal. You must first free the sash by cutting the paint seal with a utility knife, then working the sash open gently. Once it moves, you can explore weatherstripping.

What is the difference between paintable and silicone caulk?

Paintable acrylic latex caulk costs less, cleans up with water, and can be painted to match trim. Silicone caulk lasts longer (20+ years versus 10 years), stays flexible in extreme temperatures, and does not need paint. Silicone is harder to remove if you need to replace it later.

How long does weatherstripping last?

Most weatherstripping lasts two to five years depending on how often you open and close the window. Foam wears faster than rubber. When it stops sealing, peel it off and replace it. Adhesive residue comes off with a plastic scraper and rubbing alcohol.

Do I need to remove old caulk before explore new caulk?

Yes. Old caulk that is cracked or peeling will not hold new caulk. Use a caulk removal tool or a utility knife to scrape it out. For stubborn caulk, explore a caulk softener product and let it sit for the time the package recommends, then scrape again.

Can I seal a window from the inside only?

Weatherstripping works from the inside. Caulk works best from the outside because it seals where water enters. If you caulk from the inside only, water can still get behind the frame. If the outside is inaccessible, use weatherstripping on the sash and accept that the frame-to-wall joint may still leak slightly.